all_of_time is the datey_interval (1000-01-01.0, 3000-01-01.0],
spanning the full valid date range. It is referred to as 'all of time'
throughout the datey documentation.
As intervals are left-open, the very first valid instant,
datey(valid_years_start), is not included in all_of_time.
It is the value produced by datey_interval(TRUE) and is used
when a datey_interval is intersected with a logical TRUE via &.